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Deep Inelastic Scattering (DIS): Parton Distribution Functions and low-x physics Goa Sept 2008

Deep Inelastic Scattering (DIS): Parton Distribution Functions and low-x physics Goa Sept 2008 A.M.Cooper-Sarkar Oxford What have we learnt from DIS in the last 30 years? QPM, QCD Parton Distribution Functions, α s Low-x physics.

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Deep Inelastic Scattering (DIS): Parton Distribution Functions and low-x physics Goa Sept 2008

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  1. Deep Inelastic Scattering (DIS): Parton Distribution Functions and low-x physics Goa Sept 2008 A.M.Cooper-Sarkar Oxford What have we learnt from DIS in the last 30 years? QPM, QCD Parton Distribution Functions, αs Low-x physics

  2. PDFs were first investigated in deep inelastic lepton-hadron scatterning -DIS Leptonic tensor - calculable 2 Lμν Wμν dσ~ Hadronic tensor- constrained by Lorentz invariance E’ q = k – k’, Q2 = -q2 Px = p + q , W2 = (p + q)2 s= (p + k)2 x = Q2 / (2p.q) y = (p.q)/(p.k) W2 = Q2 (1/x – 1) Q2 = s x y Ee Ep s = 4 Ee Ep Q2 = 4 Ee E’ sin2θe/2 y = (1 – E’/Ee cos2θe/2) x = Q2/sy The kinematic variables are measurable

  3. Completely generally the double differential cross-section for e-N scattering d2(e±N) = [ Y+ F2(x,Q2) - y2 FL(x,Q2) ± Y_xF3(x,Q2)], Y± = 1 ± (1-y)2 dxdy Leptonic part hadronic part F2, FL and xF3 are structure functionswhich express the dependence of the cross-section on the structure of the nucleon– The Quark-Parton model interprets these structure functions as related to the momentum distributions of quarks or partons within the nucleon AND the measurable kinematic variablex = Q2/(2p.q)is interpreted as the FRACTIONAL momentum of the incoming nucleon taken by the struck quark (xP+q)2=x2p2+q2+2xp.q ~ 0 for massless quarks and p2~0so x = Q2/(2p.q) The FRACTIONAL momentum of the incoming nucleon taken by the struck quark is the MEASURABLE quantity x

  4. Consider electron muon scattering ds = 2pa2s [ 1 + (1-y)2] , for elastic eμ dy Q4 isotropic non-isotropic ds = 2pa2ei2 s [ 1 + (1-y)2] , so for elastic electron quark scattering, quark charge ei e dy Q4 d2s= 2pa2 s [ 1 + (1-y)2] Σiei2(xq(x) + xq(x)) so for eN, where eq has c. of m. energy2 equal toxs, and q(x) gives probability that such a quark is in the Nucleon dxdy Q4 Now compare the general equation to the QPM prediction to obtain the results F2(x,Q2) = Σiei2(xq(x) + xq(x)) – Bjorken scaling – this depends only on x FL(x,Q2) = 0 - spin ½ quarks xF3(x,Q2) = 0 - only γexchange

  5. Consider n,n scattering: neutrinos are handed ds(n)= GF2 x s ds(n) = GF2 x s (1-y)2 Compare to the general form of the cross-section for n/n scattering via W+/- FL(x,Q2) = 0 xF3(x,Q2) = 2Σix(qi(x) - qi(x)) Valence F2(x,Q2) = 2Σix(qi(x) + qi(x)) Valence and Sea And there will be a relationship between F2eN and F2nN Also NOTE n,n scattering is FLAVOUR sensitive dy p dy p For nq (left-left) For n q (left-right) d2s(n) = GF2 s Σi [xqi(x) +(1-y)2xqi(x)] dxdy p For nN d2s(n) = GF2 s Σi [xqi(x) +(1-y)2xqi(x)] dxdy p For nN Clearly there are antiquarks in the nucleon 3 Valence quarks plus a flavourless qq Sea m- W+ can only hit quarks of charge -e/3 or antiquarks -2e/3 n W+ u d s(np)~ (d + s) + (1- y)2 (u + c) s(np) ~ (u + c) (1- y)2 + (d + s) q = qvalence +qsea q = qsea qsea= qsea

  6. So in n, n scattering the sums over q, qbar ONLY contain the appropriate flavours BUT- high statistics n, n data are taken on isoscalar targets e.g. Fe ~ (p + n)/2=N d in proton = u in neutron u in proton = d in neutron GLS sum rule Total momentum of quarks A TRIUMPH (and 20 years of understanding the c c contribution)

  7. BUT – Bjorken scaling is broken F2 does not depend only on x it also depends on Q2 – ln(Q2) Particularly strongly at small x

  8. QCD improves the Quark Parton Model What if or x x Pqq Pgq y y Before the quark is struck? Pqg Pgg y > x, z = x/y So F2(x,Q2) = Σi ei2(xq(x,Q2) + xq(x,Q2)) in LO QCD The theory predicts the rate at which the parton distributions (both quarks and gluons) evolve with Q2- (the energy scale of the probe) -BUT it does not predict their shape The DGLAP parton evolution equations

  9. What if higher orders are needed? Note q(x,Q2) ~ αs lnQ2, but αs(Q2)~1/lnQ2, so αs lnQ2 is O(1), so we must sum all terms αsn lnQ2n Leading Log Approximation x decreases from ss(Q2) target to probe xi-1> xi > xi+1…. Pqq(z) = P0qq(z) + αs P1qq(z) +αs2 P2qq(z) LO NLO NNLO pt2 of quark relative to proton increases from target to probe pt2i-1 < pt2i < pt2i+1 Dominant diagrams have STRONG pt ordering F2 is no longer so simply expressed in terms of partons - convolution with coefficient functions is needed – but these are calculable in QCD FL is no longer zero.. And it depends on the gluon

  10. How do we determine Parton Distribution Functions ? Parametrise the parton distribution functions (PDFs) at Q20 (~1-7 GeV2)- Use NLO QCD DGLAP equations to evolve these PDFs to Q2 >Q20 Construct the measurable structure functions and cross-sections by convoluting PDFs with coefficient functions: make predictions for ~2000 data points across the x,Q2 plane- Perform χ2 fit to the data Who? Alekhin, CTEQ, MRST, H1, ZEUS http://durpdg.dur.ac.uk/hepdata/ http://projects.hepforge.org/lhapdf/ Formalism NLO DGLAP MSbar factorisation Q02 functional form @ Q02 sea quark (a)symmetry etc. fi (x,Q2) fi (x,Q2) αS(MZ ) Data DIS (SLAC, BCDMS, NMC, E665, CCFR, H1, ZEUS, CCFR, NuTeV ) Drell-Yan (E605, E772, E866, …) High ET jets (CDF, D0) W rapidity asymmetry (CDF) etc. LHAPDFv5

  11. The DATA – the main contribution is DIS data Terrific expansion in measured range across the x, Q2 plane throughout the 90’s HERA data Pre HERA fixed target p,D NMC, BDCMS, E665 and ,bar Fe CCFR We have to impose appropriate kinematic cuts on the data so as to remain in the region when the NLO DGLAP formalism is valid • Q2 cut : Q2 > few GeV2 so that perturbative QCD is applicable- αs(Q2) small • W2 cut: to avoid higher twist terms- usual formalism is leading twist • x cut: to avoid regions where ln(1/x) resummation (BFKL) and non-linear effects may be necessary….

  12. Need to extend the formalism? Optical theorem 2 The handbag diagram- QPM Im QCD at LL(Q2) Ordered gluon ladders (αsn lnQ2 n) NLL(Q2) one rung disordered αsn lnQ2 n-1 ? BUT what about completely disordered Ladders? at small x there may be a need for BFKLln(1/x) resummation? And what about Higher twist diagrams ? Are they always subdominant? Important at high x, low Q2

  13. The strong rise in the gluon density at small-x leads to speculation that there may be a need for non-linear equations?- gluons recombining gg→g Non-linear fan diagrams form part of possible higher twist contributions at low x- no sign of these in data

  14. The CUTS In practice it has been amazing how low in Q2 the standard formalism still works- down to Q2 ~ 1 GeV2 : cut Q2 > 2 GeV2 is typical It has also been surprising how low in x – down to x~ 10-5 : no x cut is typical Nevertheless there are doubts as to the applicability of the formalism at such low-x.. (See later) there could beln(1/x) corrections and/or non-linear high density corrections for x < 5 10 -3

  15. Higher twist terms can be important at low-Q2 and high-x→ this is the fixed target region (particularly SLAC- and now JLAB data). Kinematic target mass corrections and dynamic contributions ~ 1/Q2 X→ 2x/(1 + √(1+4m2x2/Q2)) Fit with F2=F2LT (1 +D2(x)/Q2) Fits establish that higher twist terms are not needed if W2 > 15 GeV2 – typical W2 cut

  16. The form of the parametrisation Parametrise the parton distribution functions (PDFs) at Q20 (~1-7 GeV2) Parameters Ag, Au, Ad are fixed through momentum and number sum rules– other parameters may be fixed by model choices- Model choices Form of parametrization at Q20, value ofQ20,, flavour structure of sea, cuts applied, heavy flavour scheme → typically ~15-22 parameters Use QCD to evolve these PDFs to Q2 >Q20 Construct the measurable structure functions by convoluting PDFs with coefficient functions: make predictions for ~2000 data points across the x,Q2 plane Perform χ2 fit to the data xuv(x) =Auxau (1-x)bu (1+ εu√x + γu x) xdv(x) =Adxad (1-x)bd (1+ εd√x + γdx) xS(x) =Asx-λs (1-x)bs (1+ εs√x + γsx) xg(x) =Agx-λg(1-x)bg (1+ εg√x + γgx) These parameters control the low-x shape These parameters control the middling-x shape These parameters control the high-x shape Alternative form for CTEQ xf(x) = A0xA1(1-x)A2 eA3x (1+eA4x)A5 The fact that so few parameters allows us to fit so many data points established QCD as the THEORY OF THE STRONG INTERACTION and provided the first measurements of s (as one of the fit parameters)

  17. The form of the parametrisationat Q20 Ultimately we may get this from lattice QCD, or other models- the statistical model is quite successful (Soffer et al). But we can make some guesses at the basic form: xa (1-x)b ….. at one time (20 years ago?) we thought we understood it! --------the high x power from counting rules ----(1-x)2ns-1 - ns spectators valence (1-x)3, sea (1-x)7, gluon (1-x)5 --------the low-x power from Regge – low-x corresponds to high centre of massenergy for the virtual boson proton collision (x = Q2 / (2p.q)) -----Regge theory gives high energy cross-sections as s (α-1) -----------which gives x dependence x (1-α), where α is the intercept of the Regge trajectory- different for singlet (no overall flavour) F2 ~x0 and non-singlet (flavour- valence-like) xF3~x0.5 The shapes of F2 and xF3 even looked as if they followed these shapes – pre HERA

  18. But at what Q2 would these be true? – Valence distributions evolve slowly but sea and gluon distributions evolve fast. We input non-perturbative ideas about shapes at a low scale, but we are just parametrising our ignorance. It turns out that we need the arbitrary polynomial In any case the further you evolve in Q2 the less the parton distributions look like the low Q2 inputs and the more they are determined by QCD evolution –Valence distributions evolve slowly Sea/Gluon distributions evolve fast (Some people don’t use a starting parametrization at all- but let neural nets learn the shape of the data- NNPDF)

  19. Where is the information coming from? • Originally- pre HERA • Fixed target e/μ p/D data from NMC, BCDMS, E665, SLAC • F2(e/p)~ 4/9 x(u +ubar) +1/9x(d+dbar) + 4/9 x(c +cbar) +1/9x(s+sbar) • F2(e/D)~5/18 x(u+ubar+d+dbar) + 4/9 x(c +cbar) +1/9x(s+sbar) • Also use ν, νbar fixed target data from CCFR( now also NuTeV/Chorus) (Beware Fe target needs nuclear corrections) • F2(ν,νbar N) = x(u +ubar + d + dbar + s +sbar + c + cbar) • xF3(ν,νbar N) = x(uv + dv ) (provided s = sbar) • Valence information for 0< x < 1 • Can get ~4 distributions from this: e.g. u, d, ubar, dbar – but need assumptions • like q=qbar for all flavours, sbar = 1/4 (ubar+dbar) and heavy quark treatment. • Note gluon enters only indirectly via DGLAP equations for evolution Assuming u in proton = d in neutron – strong-isospin

  20. Flavour structure Historically an SU(3) symmetric sea was assumed u=uv+usea, d=dv+dsea usea= ubar = dsea = dbar = s = sbar =K and c=cbar=0 Measurements of F2μn = uv + 4dv +4/3K F2μp 4uv+ dv +4/3K Establish no valence quarks at small-x F2μn/F2μp→1 But F2μn/F2μp →1/4 as x → 1 Not to 2/3 as it would for dv/uv=1/2, hence it look s as if dv/uv →0 as x →1 i.e the dv momentum distribution is softer than that of uv- Why? Non-perturbative physics --diquark structures? How accurate is this? Could dv/uv →1/4 (Farrar and Jackson)?

  21. Flavour structure in the sea dbar ≠ubar in the sea Consider the Gottfried sum-rule (at LO) ∫ dx (F2p-F2n) = 1/3 ∫dx (uv-dv) +2/3∫dx(ubar-dbar) If ubar=dbar then the sum should be 0.33 the measured value from NMC = 0.235 ± 0.026 Clearly dbar > ubar…why? low Q2 non-perturbative effects, Pauli blocking, p →nπ+,pπ0,Δ++π- m- sbar≠(ubar+dbar)/2, in fact sbar ~ (ubar+dbar)/4 Why? The mass of the strange quark is larger than that of the light quarks Evidence – neutrino opposite sign dimuon production rates And even s≠sbar? Because of p→ΛK+ n W+ c→s μ+ν s

  22. So what did HERA bring? Low-x – within conventional NLO DGLAP Before the HERA measurements most of the predictions for low-x behaviour of the structure functions and the gluon PDF were wrong HERA ep neutral current (γ-exchange) data give much more information on the sea and gluon at small x….. xSea directly from F2, F2 ~ xq xGluon from scaling violations dF2 /dlnQ2 – the relationship to the gluon is much more direct at small-x, dF2/dlnQ2 ~ Pqgxg

  23. Low-x t = ln Q2/2 Gluon splitting functions become singular αs ~ 1/ln Q2/2 At small x, small z=x/y A flat gluon at low Q2 becomes very steep AFTER Q2 evolution AND F2 becomes gluon dominated F2(x,Q2) ~ x -λs, λs=λg -ε xg(x,Q2) ~ x -λg And yet people didn’t expect this…. See later

  24. High Q2 HERA data-still to be fully exploited HERA data have also provided information at high Q2→ Z0 and W+/- become as important as γexchange → NC and CC cross-sections comparable For NC processes F2= i Ai(Q2) [xqi(x,Q2) + xqi(x,Q2)] xF3= iBi(Q2) [xqi(x,Q2) - xqi(x,Q2)] Ai(Q2) = ei2 – 2 eivi vePZ + (ve2+ae2)(vi2+ai2) PZ2 Bi(Q2) = – 2 eiai ae PZ + 4ai ae vi ve PZ2 PZ2 = Q2/(Q2 + M2Z) 1/sin2θW a new valence structure function xF3 due to Z exchange is measurable from low to high x- on a pure proton target → no heavy target corrections- no assumptions about strong isospin

  25. CC processes give flavour information d2(e+p) = GF2 M4W [x (u+c) + (1-y)2x (d+s)] d2(e-p) = GF2 M4W [x (u+c) + (1-y)2x (d+s)] dxdy 2x(Q2+M2W)2 dxdy 2x(Q2+M2W)2 uv at high x dv at high x MW information Measurement of high-x dvon a pure proton target d is not well known because u couples more strongly to the photon. Historically information has come from deuterium targets –but even Deuterium needs binding corrections. Open questions: does u in proton = d in neutron?, does dv/uv  0, as x  1?

  26. How has our knowledge evolved?

  27. To reveal the difference in both large and small x regions To view small and large x in one plot The u quark LO fits to early fixed-target DIS data

  28. Rise from HERA data

  29. The story about the gluon is more complex Gluon

  30. Gluon HERA steep rise of F2 at low x

  31. Gluon More recent fits with HERA data- steep rise even for low Q2 ~ 1 GeV2 Tev jet data Does gluon go negative at small x and low Q?seeMRST/W PDFs

  32. PDF comparison 2008 The latest HERAPDF0.1 gives very small experimental errors and modest model errors

  33. Modern analyses assess PDF uncertainties within the fit Clearly errors assigned to the data points translate into errors assigned to the fit parameters -- and these can be propagated to any quantity which depends on these parameters— the parton distributions or the structure functions and cross-sections which are calculated from them < б2F > = Σj Σk ∂ F Vjk ∂ F ∂ pj ∂ pk The errors assigned to the data are both statistical and systematic and for much of the kinematic plane the size of the point-to-point correlated systematic errors is ~3 times the statistical errors This must be treated carefully in the χ2 definition

  34. Some data sets incompatible/only marginally compatible? One could restrict the data sets to those which are sufficiently consistent that these problems do not arise – (H1, GKK, Alekhin) But one loses information since partons need constraints from many different data sets – no-one experiment has sufficient kinematic range / flavour information (though HERA comes close) To illustrate: the χ2 for the MRST global fit is plotted versus the variation of a particular parameter (αs ). The individual χ2e for each experiment is also plotted versus this parameter in the neighbourhood of the global minimum. Each experiment favours a different value of. αs PDF fitting is a compromise. Can one evaluate acceptable ranges of the parameter value with respect to the individual experiments?

  35. illustration foreigenvector-4 CTEQ6 look at eigenvector combinations of their parameters rather than the parameters themselves. They determine the 90% C.L. bounds on the distance from the global minimum from ∫ P(χe2, Ne) dχe2 =0.9 for each experiment Distance This leads them to suggest a modification of the χ2 tolerance, Δχ2 = 1, with which errors are evaluated such that Δχ2 = T2, T = 10. Why? Pragmatism. The size of the tolerance T is set by considering the distances from the χ2 minima of individual data sets from the global minimum for all the eigenvector combinations of the parameters of the fit. All of the world’s data sets must be considered acceptable and compatible at some level, even if strict statistical criteria are not met, since the conditions for the application of strict statistical criteria, namely Gaussian error distributions are also not met. One does not wish to lose constraints on the PDFs by dropping data sets, but the level of inconsistency between data sets must be reflected in the uncertainties on the PDFs.

  36. Recent development: Combining ZEUS and H1 data sets • Not just statistical improvement. Each experiment can be used to calibrate the othersince they have rather different sources of experimental systematics • Before combination the systematic errors are ~3 times the statistical for Q2< 100 • After combination systematic errors are < statistical • → very consistent data input HERAPDFs use Δχ2=1

  37. Diifferent uncertainty estimates on the gluon persist as Q2 increases Q2=10 Note CTEQ in general bigger uncertainties…but NOT for low-x gluon CTEQ6.5 MSTW08 Q2=10000

  38. The general trend of PDF uncertainties for conventional NLO DGLAP is that The u quark is much better known than the d quark The valence quarks are much better known than the gluon/sea at high-x The valence quarks are poorly known at small-x but they are not important for physics in this region The sea and the gluon are well known at low-x but onlydown to x~10-4 The sea is poorly known at high-x, but the valence quarks are more important in this region The gluon is poorly known at high-x And it can still be very important for physics

  39. End lecture 1 • Next PDFs for the LHC • Be-(a)ware of low-x physics

  40. The Standard Model is not as well known as you might think fa pA x1 • where X=W, Z, D-Y, H, high-ET jets, prompt-γ • and  is known • to some fixed order in pQCD and EW • in some leading logarithm approximation (LL, NLL, …) to all orders via resummation ^ pB x2 fb X Knowledge from HERA →the LHC- transport PDFs to hadron-hadron cross-sections using QCD factorization theorem for short-distance inclusive processes The central rapidity range for W/Z production AT LHC is at low-x (5 ×10-4 to 5 ×10-2)

  41. MRST PDF NNLO corrections small ~ few% NNLO residual scale dependence < 1% W/Z production have been considered as good standard candle processes with small theoretical uncertainty. PDF uncertainty is THE dominant contribution and most PDF groups quote uncertainties <~5% (but note HERAPDF ~1-2%) BUTthe central values differ by more than some of the uncertainty estimates.

  42. Look at predictions for W/Z rapidity distributions Pre- and Post-HERA Why such an improvement? Pre HERA Post HERA It’s due to the improvement in the low-x gluon At the LHC the q-qbar which make the boson are mostly sea-sea partons at low-x And at Q2~MZ2 the sea is driven by the gluon

  43. This is post HERA but just one experiment This is post HERA using the new (2008) HERA combined PDF fit However there is still the possibility of trouble with the formalism at low-x

  44. Moving on to BSM physics Example of how PDF uncertainties matter for BSM physics– Tevatron jet data were originally taken as evidence for new physics-- i These figures show inclusive jet cross-sections compared to predictions in the form (data - theory)/ theory Something seemed to be going on at the highest E_T And special PDFs like CTEQ4/5HJ were tuned to describe it better- note the quality of the fits to the rest of the data deteriorated. But this was before uncertainties on the PDFs were seriously considered

  45. Today Tevatron jet data are considered to lie within PDF uncertainties. (Example from CTEQ hep-ph/0303013) We can decompose the uncertainties into eigenvector combinations of the fit parameters-the largest uncertainty is along eigenvector 15 –which is dominated by the high x gluon uncertainty

  46. SM 2XD 4XD 6XD Such PDF uncertainties on the jet cross sections compromise the potential for discovery of any physics effects which can be written as a contact interaction E.G. Dijet cross section potential sensitivity to compactification scale of extra dimensions (Mc) reduced from ~6 TeV to 2 TeV. Mc = 2 TeV, no PDF error Mc = 6 TeV, no PDF error Mc = 2 TeV, with PDF error

  47. BEWARE of different sort of ‘new physics’ LHC is a low-x machine (at least for the early years of running) Low-x information comes from evolving the HERA data Is NLO (or even NNLO) DGLAP good enough? The QCD formalism may need extending at small-x BFKL ln(1/x) resummation High density non-linear effects etc. (Devenish and Cooper-Sarkar, ‘Deep Inelastic Scattering’, OUP 2004, Section 6.6.6 and Chapter 9 for details!)

  48. Before the HERA measurements most of the predictions for low-x behaviour of the structure functions and the gluon PDF were wrong Now it seems that the conventional NLO DGLAP formalism works TOO WELL _ there should beln(1/x) corrections and/or non-linear high density corrections for x < 5 10 -3

  49. Low-x t = ln Q2/2 Gluon splitting functions become singular αs ~ 1/ln Q2/2 At small x, small z=x/y A flat gluon at low Q2 becomes very steep AFTER Q2 evolution AND F2 becomes gluon dominated F2(x,Q2) ~ x -λs, λs=λg -ε xg(x,Q2) ~ x -λg

  50. So it was a surprise to see F2 steep at small x - for low Q2, Q2 ~ 1 GeV2 Should perturbative QCD work? αs is becoming large - αs at Q2 ~ 1 GeV2 is ~ 0.4

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